How can I randomize object instance's rotation?
It's pretty straightforward once you get ahold of the fcurves, which is the data used by Blender to animate properties.

This node tree works only for animated rotation, but you can easily expand it to take into account scale or location animations, by accessing the other fcurves.

First, add a group which we will use to extract each fcurve data at a given frame, and add an offset to it, with an integer as a parameter to randomize it for each individual cube.



Important :


Set the "Random Number" node to min =  - your max frame number (in my case -100) and max = min frame number (here, 0)
Set the "Repeat time" node to loop above your max frame number.
Set the "Evaluate Fcruve" node to Absolute


Then, simply loop over each object and gather the result of each fcurve, then assign it to your object's rotation :



Note that the rotations are in Radians.

My main tree looks like this :



Result :